The present invention relates generally to a well-cementing stage collar, and more specifically to such a stage collar operable in two operating modes for initially exposing vent holes prior to a desired cementing operation, and subsequently covering those holes on completion of the cementing operation.
In preparing well bore holes for oil and/or gas production, it is necessary to insert cement into the annular space between the casing and the bore hole. This may be done for various reasons, including isolating undesired regions or, correspondingly, isolating gas or oil production zones. During cementing, a cement slurry is passed down through the casing to critical points, or stages, in the bore hole.
Multiple stage cementing is achieved by placing cementing tools, more commonly referred to as stage collars, at more than one location in the bore hole. The stage collars are selectively operable as valves to provide for the passage of cement through ports located in the collar. Most stage collars include upper and lower sleeves which are slidable inside the collar. The upper sleeve has a larger inner diameter than the lower sleeve.
Typically, both sleeves are initially fastened to the collar by shear pins, with the lower sleeve covering the ports. When cementing is desired, a trip bomb sized to pass through the upper sleeve but not the lower sleeve, is forced down the casing until it seats on the lower sleeve. Sufficient fluid pressure is then applied behind the plug to shear the pins holding the lower sleeve. This moves the lower sleeve down to a point where the cementing ports are uncovered. The bomb forms a block in the casing causing cement slurry then pumped into the casing to pass out the ports into the annulus.
After cementing is completed, a second bomb is passed down the casing until it seats on the upper sleeve. Appropriate fluid pressure is applied behind the plug to shear its pins. This allows the upper sleeve to move down and close off the inside of the ports.
Stage collars of the type just described frequently have operating problems. Among them is the problem of cement slurry in the annulus putting substantial hydrostatic pressure through the ports on the upper sleeve when it is in the closed position, prior to setting of the cement. This can allow cement to seep back into the inside of the casing where it can cause obstacles and even plugs to form when the cement sets. In order to avoid this problem, it is desirable to provide a cover the outside of the port. This has been provided by one device known to applicant in which a hydraulic valve is opened when the second sleeve drops down into position as described above. With this valve open, hydraulic pressure applied inside the casing causes an outside member to shear holding pins and to be hydraulically driven down along the outer wall. If it works properly, this can solve the above-described problem. However, it too can have various operating problems. In order for it to work properly, the second sleeve must properly clear the appropriate valve opening and the limited valve passageway which is typically required may become plugged or obstructed from substances in the well casing. In addition, it requires additional operating costs because it involves a third and separate step prior to setting of the cement in order to accomplish the complete closing operation.
It is therefore a general object of the present inventoin to overcome these disadvantages of the prior art.
Specifically, it is desired to provide a device which is operable in a two-step operation to open the stage collar ports and close them from the outside.
It is an additional objective to provide a collar which simultaneously closes both the inside and outside port openings.
It is a further desired objective to provide a cementing collar which has limited moving parts and is simple to operate either hydraulically or mechanically.
A stage collar made according to the present invention uses an upper and lower internal sliding sleeve to initially open and then close the cementing ports from the inside. Additionally, it includes an outer slide member associated with each port which is attached fixedly to the upper sleeve through appropriate apertures in the collar body. During the port-closing step, the upper inner sleeve and the outer slides slide down unitarily to cover simultaneously the inside and outside of the cementing ports.
It can be seen that such a device provides for simple operation while providing for complete closure of the cementing ports. These and additional objects and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following detailed description of the preferred embodiment.